Amid a crescendo of complaints from Russians fed up with the country’s tightly controlled political system, two prominent figures — a billionaire industrialist and the recently ousted finance minister — sought to fill a void in the opposition leadership on Monday.

The billionaire, Mikhail D. Prokhorov, who owns shares in a major gold mining company and an array of other ventures in Russia as well as the New Jersey Nets basketball franchise in the United States, said he will run for president, challenging Prime Minister Vladimir V. Putin.

“I made a decision, probably the most serious decision in my life: I am going to the presidential election,” Mr. Prohkorov said at a news conference. He has barely appeared in public since mid-September, when he was dramatically removed as the head of a pro-business party, Just Cause, after clashing with Kremlin political strategists.

“You may remember, the Kremlin removed me and my allies from Just Cause, and we were not allowed to do what we wanted,” he said. “It is not in my nature to stop halfway. So for the last two and a half months we sat and worked, very calmly and quietly, and we created all the infrastructure to collect two million signatures,” which are needed to get on the ballot.